As a gas sensor for performing the enhancement of fuel economy and combustion control of an internal combustion engine of an automobile or the like, there has been known an oxygen sensor or an air/fuel sensor which detects the oxygen concentration in an exhaust gas. Further, along with the strengthening of the regulations on an exhaust gas from an automobile, there has been a demand for the reduction of an amount of nitrogen oxide (NOx) in the exhaust gas, and a NOx sensor which can directly measure the concentration of NOx has been developed.
As such a NOx sensor, there has been known a sensor which includes a NOx sensor element having three cells each of which includes a pair of electrodes formed on a surface of oxygen-ion conductive solid electrolyte made of zirconia or the like. The NOx sensor element extends in the axial direction, and is constituted of a first pumping cell, an oxygen concentration detection cell and a second pumping cell arranged sequentially from a distal end side as viewed along the axial direction, and the periphery of the NOx sensor element is held by a body fitting.
A first measuring chamber into which a gas to be measured is introduced is defined in the NOx sensor, and the first pumping cell is arranged so as to face the first measuring chamber. By performing an energization control of the first pumping cell by a sensor control apparatus to which the NOx sensor is connected, the first pumping cell is driven so as to pump out oxygen from the first measuring chamber or to pump oxygen into the first measuring chamber from the outside. The oxygen concentration detection cell is provided in such a manner that the oxygen concentration detection cell is laminated to the first pumping cell and faces the first measuring chamber. A voltage (electric motive force) corresponding to the oxygen concentration in the first measuring chamber is outputted by the oxygen concentration detection cell. The energization control of the first pumping cell is performed such that the voltage outputted by the oxygen concentration detection cell becomes a predetermined voltage, and the oxygen concentration in a gas to be measured in the first measuring chamber is controlled to the predetermined concentration by driving the first pumping cell. Here, an electric current which flows in the first pumping cell is correlated with the oxygen concentration in the gas to be measured and hence, the oxygen concentration in the gas to be measured can be detected based on the electric current.
Then, the gas to be measured whose oxygen concentration is controlled flows into a NOx measuring chamber (second measuring chamber) which is communicated with the first measuring chamber. By applying a fixed voltage to the second pumping cell which faces the NOx measuring chamber, NOx in the gas to be measured is decomposed so that an electric current corresponding to the NOx concentration flows between a pair of electrodes of the second pumping cell whereby the NOx concentration can be detected based on this electric current.
Here, to accurately detect the oxygen concentration and the NOx concentration, it is necessary to activate each cell by heating the NOx sensor to a predetermined activation temperature (for example, a predetermined temperature of 750° C. or above) and hence, a heater is provided to the NOx sensor as an integral part of each cell, and the NOx sensor is connected to a control apparatus which performs an energization control of the heater. With respect to such a NOx sensor control apparatus, there has been developed a technique where impedance (inner resistance) of one cell which constitutes a NOx sensor is measured and an energization state of the heater is controlled such that measured impedance becomes preset target impedance (see patent document 1). Usually, the NOx sensor control apparatus, while performing such a control of the energization state of the heater, performs a control of an energization state of each cell, and also converts an electric current which flows into the second pumping cell into a voltage, calculating a detection value of NOx concentration (concentration converted value) based on the voltage, and outputting the detected value to an external engine control apparatus.